Fuel cells are energy conversion systems that convert chemical energies of fuels directly into electric energies. Fuel cells have high energy efficiency, and are environmentally friendly in that they are substantially free from emission of pollutants. Therefore, there have been many studies about fuel cells as alternative energy sources.
Among such fuel cells, polymer electrolyte membrane fuel cells (PEMFCs) are particularly advantageous, because they have low drive temperature, are free from the leakage problem by the use of a solid electrolyte, and allow high-speed operation. Thus, PEMFCs have been spotlighted as portable, automotive and household power sources. Additionally, as compared to other types of fuel cells, PEMFCs provide high output under high current density, can be operated at a temperature lower than 100° C., have a simple structure, exhibit high initiation and response characteristics, and show excellent durability. In addition to the above, PEMFCs allow the use of hydrogen gas, methanol or natural gas as fuel. Further, continuous research and development has been conducted to provide PEMFCs as portable fuel cells because of their high output density permitting significant size reduction.
An ion exchange membrane, used as a solid electrolyte in a fuel cell, is interposed between both electrodes and allows protons generated at an anode to move toward a cathode. Herein, a polymer membrane having a sulfonic acid group (—SO3H) introduced thereto is widely used as an electrolyte.
In general, electrolytes used in PEMFCs may be divided into perfluorinated polymer electrolytes and hydrocarbon-based polymer electrolytes. The perfluorinated polymer electrolytes are chemically stable due to a strong binding force between carbon and fluorine (C—F) and a so-called shielding effect unique to fluorine atoms, and have excellent mechanical properties. Particularly, perfluorinated polymer electrolytes show high conductivity as proton exchange membranes, and thus are widely used as polymer electrolyte membranes in PEMFCs. As a typical example, Nafion, a perfluorinated polymer membrane developed by Dupont Inc. has been widely used as a proton exchange membrane in the field of fuel cells, because it shows excellent ion conductivity, chemical stability and ion selectivity. However, such perfluorinated polymer electrolyte membranes are problematic in that they are expensive in spite of their excellent quality, thereby degrading their industrial applicability, they cause a significant methanol crossover phenomenon including permeation of methanol through a polymer membrane, and they show a drop in efficiency of a polymer membrane at a temperature of 80° C. or higher. Therefore, there have been intensive studies about hydrocarbon-based ion exchange membranes, which show high cost-efficiency.
A polymer electrolyte membrane used in a fuel cell should be stable under the conditions required for driving the fuel cells. Thus, such polymer electrolyte membranes are required to have sufficient mechanical properties. When increasing the thickness of a membrane in order to improve mechanical properties, the membrane has an increased resistance, resulting in an undesired drop in ion conductivity of the membrane. When a fuel cell drives, the above poor-quality polymer membrane is decomposed due to electrochemical stresses including hydrolysis, oxidation and reduction, resulting in degradation in quality of the fuel cell. Additionally, a polymer electrolyte membrane for a fuel cell absorbs a significant amount of water in its hydrophilic domain, when a fuel cell drives. Water content of a polymer membrane has an effect on ion conductivity, mechanical stability and gas barrier property of an electrolyte membrane. Also, since a polymer electrolyte membrane has anisotropic characteristics, longitudinal expansion of a polymer electrolyte membrane generated during the hydration thereof depends not only on the humidity of the membrane but also on the alignment direction of the polymer. Further, mechanical properties and ion conductivity of a polymer electrolyte membrane vary with the alignment direction of the polymer. Therefore, there has been increasing interest in a reinforced composite polymer electrolyte membrane, which has a reduced thickness and resistance so as to increase ion conductivity while improving the dimensional stability thereof.
U.S. Pat. Nos. 5,547,551 and 5,559,614 disclose a process for manufacturing a composite electrolyte membrane having a thickness of 25 μm or less, the process comprising the steps of providing a microporous polytetrafluoroethylene film supported on polymer fibrils and filling micropores of the membrane with an ion exchange material. However, the above process for manufacturing a composite electrolyte membrane requires the use of a surfactant in order to prevent a phase separation phenomenon occurring between the microporous polytetrafluoroethylene film and the ion exchange material, and thus is problematic in that it requires a complicated post-treatment step of completely removing the surfactant and the portion, from which the surfactant is removed, may adversely affect driving of a fuel cell.